Gas separation techniques using a membrane are rapidly developing their applications because of economy of energy, high safety and simpleness in handling. Among them, the technique with which oxygen is separated from other members in a mixture of gases is useful, particularly when the mixture of gases is the air and the technique is employed to concentrate oxygen. If an oxygen-enriched air could be produced from air in a simple and economical way, a large contribution is expected in many fields of industry, for example, a variety of combustion engines, medical instruments, food industry, and disposal of industrial exhausts.
Membranes which are employed for these purposes should desirably bear the characteristics; a large ratio of the permeability coefficients of oxygen gas upon nitrogen gas and a large permeability for oxygen gas. Especially the latter is more important, because a larger amount of oxygen per unit time permits the separation apparatus to be smaller and the amount of treated gas to be increased. A large permeation of oxygen gas can be obtained by choosing a material of a high permeability coefficient P.sub.O2 for oxygen and making the thickness of the membrane as thin as possible. Thus, the material of the membrane should have sufficient strength enough to sustain the gaseous pressure when formed in a thin film.
Among the polymer membranes ever known, polydimethylsiloxane membranes typically represent those having the largest permeability coefficient P for gases (if not otherwise specified, unit of the permeability coefficient is expressed hereinafter in cm.sup.3 (STP).cm/cm.sup.2. sec cmHg). Although the permeability coefficient P.sub.O2 for oxygen is as high as 6.times.10.sup.-8, the separation coefficient .alpha. between oxygen and nitrogen gases (the permeability coefficient P.sub.O2 for oxygen gas divided by the permeability coefficient P.sub.N2 for nitrogen gas) is as low as 2.0. Further, a membrane of polydimethylsiloxane is low in the mechanical strength and therefore a membrane not more than several ten .mu.m thick can not be employed for practical uses. Thus, polydimethylsiloxane does not provide a membrane of sufficient high permeability for oxygen in the practical sense. For improving the membrane characteristics of polydimethylsiloxane, copolymers of polydimethylsiloxane with polymers of high mechanical strength such as polycarbonate and poly-.alpha.-methylstyrene have been developed (for example, as disclosed in U.S. Pat. Nos. 3,980,456, 3,874,986 and in Japanese Laid-Open patent application No. Sho 56-26504). But result was not always satisfactory because of the lowered permeability coefficient P.sub.O2 for oxygen or the insufficient separation coefficient .alpha..
On the other hand, as a material having a higher permeability coefficient for gases than polydimethylsiloxane is known poly(disubstituted acetylene) represented by polytrimethylsilylpropyne (J. Am. Chem. Soc., 1983, 105, p.7473 and J. Appl. Polym. Sci., 1985, 30, p.1605). Membranes prepared from polytrimethylsilylpropyne have a permeability coefficient P.sub.O2 for oxygen 4-7.times.10.sup.-7 and a separation coefficient .alpha. 1.7-2.0. This material can be processed in the form of membrane due to its excellent film strength. However, if used as separation membrane for oxygen, sufficiently high oxygen concentration is not obtained owing to the small separation coefficient .alpha. and a further defect is that the permeability coefficient for oxygen is lowered during a prolonged use.
The present invention has been pursued with the aim to solve the difficulties of previous gas separation membranes, particularly of membranes prepared from poly(disubstituted acetylene), and provides a novel membrane material having sufficiently high membrane strength, excellent permeability and selectivity for gases and a stable permeability coefficient for gases.
Being interested in the excellent membrane strength and a high gas permeability coefficient of poly(disubstituted acetylene) represented by polytrimethylsilylpropyne, the present inventors looked eagerly for a novel material for membrane which assures a high separation coefficient for gases and a stable gas permeability coefficient. As a result, they found that poly(disubstituted acetylene)/polyorganosiloxane graft copolymer which was obtained by introducing polysiloxane chains into poly(disubstituted acetylene) gave a membrane having, as well as a good permeability coefficient for gases and sufficient strength, a better gas separation coefficient over that of both poly(disubstituted acetylene) and polyorganosiloxane and a stable permeability coefficient for gases. Thus, the present invention has been achieved.